Refrigeration System Having A Variable Speed Compressor

ABSTRACT

A two-stage cascade refrigeration system is provided having a first refrigeration stage and a second refrigeration stage. The first refrigeration stage defines a first fluid circuit for circulating a first refrigerant, and has a first compressor, a condenser, and a first expansion device. The second refrigeration stage defines a second fluid circuit for circulating a second refrigerant, with the second refrigeration stage having a second compressor that is a variable speed compressor, a second expansion device, and an evaporator. A heat exchanger is in fluid communication with the first and second fluid circuits to exchange heat between the first and second refrigerants. A controller stages operation of the first and second compressors and runs the second compressor at an initial speed less than a maximum speed initially when a staging protocol is performed during start up or re-starting of the refrigeration system.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of co-pending U.S. Ser. No.16/058,002, filed Aug. 8, 2018, which is a continuation of co-pendingU.S. Ser. No. 15/649,859, filed Jul. 14, 2017, which is a continuationof U.S. Ser. No. 13/196,149, filed Aug. 2, 2011, now U.S. Pat. No.9,835,360, which is a continuation of U.S. Ser. No. 12/570,348, filedSep. 30, 2009, now U.S. Pat. No. 8,011,191, the disclosures of which arehereby incorporated herein by reference in their entireties.

TECHNICAL FIELD

The present invention relates generally to refrigeration systems and,more particularly, to two-stage cascade refrigeration systems.

BACKGROUND

Two-stage cascade refrigeration systems are known for cooling spacessuch as the interior of cabinets, for example, to temperatures wellbelow zero degrees centigrade, such as temperatures below −40° C., forexample. For example, freezers of the type known as ultra-lowtemperature freezers (“ULTs”) are known to use this type ofrefrigeration system and are used to cool cabinet interiors totemperatures as low as about −80° C. or even lower.

Refrigeration systems of this type are known to include two stagescirculating respective first and second refrigerants. The first stagetransfers energy (i.e., heat) from the first refrigerant to thesurrounding environment through a condenser, while the secondrefrigerant of the second stage receives energy from the cooled space(e.g., a cabinet interior) through an evaporator. Heat is transferredfrom the second refrigerant to the first refrigerant through a heatexchanger that is in fluid communication with the two stages of therefrigeration system.

Conventional two-cascade refrigeration systems utilize compressors eachhaving a single, fixed speed, and conventionally having the same maximumcapacity. In this regard, operation of the system may entail simplyactivating and deactivating each of the two compressors at varioustimes. The ability of systems of this type to attain a uniformtemperature in the cooled space, however, is limited, and the efficiencyof operating such systems is also limited, as is the life expectancy ofthe systems themselves. In addition, operating one or both of thecompressors at maximum capacity may be detrimental, while operating oneor both of the compressors at a capacity lower than the maximum capacityfor that compressor results in operational inefficiencies. Further,conventional two-cascade refrigeration systems are known to operate at asingle predetermined level of noise during steady-state operation.

It would be desirable, therefore, to provide a refrigeration system thataddresses these and other problems associated with conventionaltwo-stage cascade refrigeration systems.

SUMMARY

In one embodiment, a two-stage cascade refrigeration system is providedhaving a first refrigeration stage and a second refrigeration stage. Thefirst refrigeration stage defines a first fluid circuit for circulatinga first refrigerant, and has a first compressor, a condenser, and afirst expansion device that is in fluid communication with the firstfluid circuit. The second refrigeration stage defines a second fluidcircuit for circulating a second refrigerant, with the secondrefrigeration stage having a second compressor, a second expansiondevice, and an evaporator that is in fluid communication with the secondfluid circuit. A heat exchanger is in fluid communication with the firstand second fluid circuits to exchange heat between the first and secondrefrigerants. At least one of the first or second compressors is avariable speed compressor.

In specific embodiments, each of the first and second compressors is avariable speed compressor. The first compressor may have a first maximumcapacity and the second compressor may have a second maximum capacity,with the second maximum capacity being, in some embodiments, less thanthe first maximum capacity, and being, in other embodiments,substantially equal to the first maximum capacity.

In embodiments where the second compressor is a variable speedcompressor, the system may include at least one controller that isoperatively coupled to the first and second compressors forindependently controlling operation of the compressors, and a sensorthat is operatively coupled to the at least one controller. The sensormay, for example, be configured to sense a temperature of the firstrefrigerant at an outlet of the heat exchanger, sense a dischargepressure of the first or second refrigerants, or sense a dischargetemperature or a suction temperature of the first refrigerant, and togenerate a signal that is indicative of the sensed temperature orpressure to the at least one controller, with the at least onecontroller being operable to vary the speed of the second compressor inresponse to the signal.

In other specific embodiments, each of the first and second compressorsis a variable speed compressor and the system includes a cabinet havingan interior and a door that provides access into the interior, and atleast one controller that is operatively coupled to the first and secondcompressors for independently controlling operation thereof. A sensor isoperatively coupled to the at least one controller and is configured tosense a condition of the door and to generate a signal that isindicative of the sensed condition to the at least one controller, withthe at least one controller being operable to vary the speed of at leastone of the first or second compressors in response to the signal. Thesystem may alternatively or additionally include a sensor that isconfigured to sense the temperature of ambient air proximate thecondenser and to generate a signal to the at least one controller thatis indicative of the sensed temperature, with the at least onecontroller being operable, in response to the signal, to vary the speedof the at least one of the first or second compressors.

In specific embodiments, the system includes a sensor that isoperatively coupled to the at least one controller and which isconfigured to sense a temperature of the first refrigerant at an outletof the heat exchanger and to generate a signal that is indicative of thesensed temperature to the at least one controller. The at least onecontroller is operable to compare the sensed temperature to apre-determined threshold temperature above which the second compressoris not activated by the at least one controller.

Additionally, the system may include a sensor that is configured tosense the temperature of ambient air proximate the condenser and togenerate a second signal indicative of the sensed temperature to the atleast one controller. The at least one controller is operable, inresponse to the second signal, to vary the pre-determined thresholdtemperature above which the second compressor is not activated by the atleast one controller.

The system may include a cabinet having an interior and a sensoroperatively coupled to the at least one controller and which isconfigured to sense the temperature of the interior of the cabinet andto generate a signal indicative of the cabinet interior temperature tothe at least one controller, with the at least one controller beingoperable, in response to this signal, to delay activation of the secondcompressor. The controller of some embodiments may vary the speed of avariable speed fan directing air across the condenser, for example, inresponse to a signal received from a sensor configured to sense thetemperature of ambient air proximate the condenser.

The system may include a pair of sensors operatively coupled to the atleast one controller and which are configured to respectively sense thedischarge pressures of the first and second refrigerants and to generaterespective signals to the at least one controller indicative of thesensed discharge pressures. The at least one controller is operable, inresponse to the signals, to vary the speed of at least one of the firstor second compressors.

The system may additionally or alternatively include a first pluralityof sensors for sensing one or more of the suction temperature, sumptemperature, discharge temperature, or discharge pressure of the firstrefrigerant, and a second plurality of sensors for sensing one or moreof the suction temperature, sump temperature, discharge temperature ordischarge pressure of the second refrigerant. The first and secondpluralities of sensors may be configured to generate respective signalsto the at least one controller which are indicative of the sensedtemperatures or pressures, with the at least one controller beingoperable, in response to the signals, to vary the speed of at least oneof the first or second compressors.

The system may also include a control interface operatively coupled tothe at least one controller for selecting among different pre-determinednoise level modes of operation of the refrigeration system. Thecontroller may include a steady-state operation mode that includessimultaneous operation of the first and second compressors.

In yet another embodiment, a method is provided for operating arefrigeration system. The method includes circulating a firstrefrigerant through a first compressor, a condenser, and a firstexpansion device of a first stage of the refrigeration system. A secondrefrigerant is circulated through a second compressor, a secondexpansion device, and an evaporator of a second stage of therefrigeration system. Heat is exchanged between the first and secondrefrigerants and the speed of at least one of the first or secondcompressors is selectively varied to control the flow of at least one ofthe first or second refrigerants.

The system disclosed herein is, accordingly, capable of attaining arelative long life expectancy, operating in an efficient manner, andattaining a uniform temperature distribution in the cooled space.Further, the system disclosed herein is capable of quickly recoveringfrom unexpected high-load conditions resulting, for example, from thestoring of a relatively warm item in the cooled space.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of this specification, illustrate embodiments of the invention and,together with a general description of the invention given above, andthe detailed description of the embodiments given below, serve toexplain the principles of the invention.

FIG. 1 is a perspective view of an exemplary refrigeration unit.

FIG. 2 is a schematic representation of a refrigeration system forcooling a cabinet interior of the unit of FIG. 1.

FIG. 3 is a schematic representation of a staging protocol for operationof the system of FIG. 2.

FIG. 4 is a schematic representation of steady-state protocol foroperation of the system of FIG. 2.

FIG. 5 is a schematic representation of another protocol for operationof the system of FIG. 2.

DETAILED DESCRIPTION

With reference to the figures, and more specifically to FIG. 1, arefrigeration unit in the form of an ultra-low temperature freezer(“ULT”) 10 is illustrated. Various aspects of the exemplary freezer 10according to one embodiment of the present invention are described andillustrated in commonly assigned U. S. patent application Ser. No.12/570,480, the disclosure of which is hereby expressly incorporated byreference herein in its entirety.

The freezer 10 of FIG. 1 includes a deck 14 that supports a cabinet 16thereabove, for storing items that require cooling to temperatures ofabout −80° C. or lower, for example. The cabinet 16, in turn, includes acabinet housing 16 a and a door 16 b providing access into an interior16 c of the cabinet 16. The deck 14 supports one or more components thatjointly define a two-stage cascade refrigeration system 20 (FIG. 2) thatthermally interacts with cabinet 16 to cool the interior 16 c thereof.

With reference to FIG. 2, a schematic representation of refrigerationsystem 20 is illustrated. System 20 is made up of a first stage 24 and asecond stage 26 respectively defining first and second circuits forcirculating a first refrigerant 34 and a second refrigerant 36. Thefirst stage 24 transfers energy (i.e., heat) from the first refrigerant34 to the surrounding environment 40, while the second refrigerant 36 ofthe second stage 26 receives energy from a cabinet interior 16 c. Heatis transferred from the second refrigerant 36 to the first refrigerant34 through a heat exchanger 44 that is in fluid communication with thefirst and second stages 24, 26 of the refrigeration system 20.

The first stage 24 includes, in sequence, a first compressor 50, acondenser 54, and a first expansion device 58. A fan 62 directs ambientair across the condenser 54 through a filter 54 a and facilitates thetransfer of heat from the first refrigerant 34 to the surroundingenvironment 40. The second stage 26 includes, also in sequence, a secondcompressor 70, a second expansion device 74, and an evaporator 78. Theevaporator 78 is in thermal communication with the interior 16 c ofcabinet 16 (FIG. 1) such that heat is transferred from the interior 16 cto the evaporator 78, thereby cooling the interior 16 c. The heatexchanger 44 is in fluid communication with the first stage 24 betweenthe first expansion device 58 and the first compressor 50. Further, theheat exchanger 44 is in fluid communication with the second stage 26between the second compressor 70 and the second expansion device 74.

In operation, the second refrigerant 36 receives heat from the interior16 c through the evaporator 78 and flows from the evaporator 78 to thesecond compressor 70 through a conduit 90. A suction/accumulator device92 is in fluid communication with the conduit 90 to pass the secondrefrigerant 36 in gaseous form to the second compressor 70, whileaccumulating excessive amounts of the same in liquid form and feeding itto the second compressor 70 at a controlled rate. From the secondcompressor 70, the compressed second refrigerant 36 flows through aconduit 96 and into the heat exchanger 44 thermally communicating thefirst and second stages 24, 26 with one another. The second refrigerant36 enters the heat exchanger 44 in gas form and transfers heat to thefirst refrigerant 34 as the second refrigerant condenses. In thisregard, the flow of the first refrigerant 34 may, for example, becounter-flow relative to the second refrigerant 36, so as to maximizethe rate of heat transfer. In one specific, non-limiting example, theheat exchanger 44 is in the form of a brazed plate heat exchanger,vertically oriented within the deck 14 (FIG. 1), and designed tomaximize the amount of turbulent flow of the first and secondrefrigerants 34, 36 within heat exchanger 44, which in turn maximizesthe heat transfer from the condensing second refrigerant 36 to theevaporating first refrigerant 34. Other types or configurations of heatexchangers are possible as well.

The second refrigerant 36 exits the heat exchanger 44, in liquid form,through an outlet 44 a thereof and flows through a conduit 102, througha filter/dryer unit 103, then through the second expansion device 74,and then back to the evaporator 78 of the second stage 26. The secondstage 26 of this exemplary embodiment also includes an oil loop 104 forlubricating the second compressor 70. Specifically, the oil loop 104includes an oil separator 106 in fluid communication with conduit 96 andan oil return line 108 directing oil back into second compressor 70.Additionally, or alternatively, the second stage 26 may include ade-superheater device 110 to cool down the discharge stream of thesecond refrigerant 36 and which is in fluid communication with conduit96 upstream of the heat exchanger 44.

As discussed above, the first refrigerant 34 flows through the firststage 24. Specifically, the first refrigerant 34 receives heat from thesecond refrigerant 36 flowing through the heat exchanger 44, exits theheat exchanger 44 in gas form through an outlet 44 b thereof and flowsthrough a pair of conduits 114, 115 towards the first compressor 50. Asuction/accumulator device 116 is positioned between conduits 114 and115 to pass the first refrigerant 34 in gaseous form to the firstcompressor 50, while accumulating excessive amounts of the same inliquid form and feeding it to the first compressor 50 at a controlledrate. From the first compressor 50, the compressed first refrigerant 34flows through a conduit 118 and into the condenser 54. The firstrefrigerant 34 in condenser 54 transfers heat to the surroundingenvironment 40 as the first refrigerant condenses before flowing inliquid form through a pair of conduits 122, 123, through a filter/dryerunit 126, and into the first expansion device 58, where the firstrefrigerant 34 undergoes a pressure drop. From the first expansiondevice 58, the first refrigerant 34 flows through a conduit 127 backinto the heat exchanger 44, entering the same in liquid form.

With continued reference to FIG. 2, at least one of the first or secondcompressors 50, 70 of this embodiment is a variable speed compressor. Ina specific embodiment, the first and second compressors 50, 70 may havedifferent maximum capacities. For example, and without limitation, thesecond compressor 70 may have a maximum capacity that is less than themaximum capacity of the first compressor 50. Alternatively, the maximumcapacities of the first and second compressors 50, 70 may besubstantially equal to one another. Moreover, operation of the system 20may be designed such that, in steady-state mode, one or both of thecompressors 50, 70 operates at the maximum capacity or at less than itsmaximum capacity, which may be desirable, for example, to maximize thelife expectancy of the compressors 50, 70.

System 20 includes an exemplary controller 130 that is operativelycoupled to each of the first and second compressors 50, 70 forindependently controlling each of the compressors 50, 70. While thisembodiment illustrates a single controller 130, those of ordinary skillin the art will readily appreciate that system 20 may have any othernumber of controllers instead. An exemplary interface 132 is operativelycoupled to the controller 130 to enable interaction with the controllerby a user. Such interaction may include, for example, choosing fromamong different modes of operation of system 20. For example, andwithout limitation, different modes of operation may be associated withdifferent maximum normally accepted noise levels of the system 20 duringsteady-state operation, such as noise standards issued by OSHA, forexample, different temperature ranges for each of the stages 24, 26,and/or different temperature settings for the cooled space (e.g.,cabinet interior 16 c). More specifically, the same freezer designed foroperation in an enclosed laboratory may be set by the user not to exceeda particular noise level (which could result in one or both compressorsbeing limited to a particular percentage of maximum speed and, if avariable speed fan is used, its speed as well). The same freezeroperated in a large area could be set or reset to allow for a higherpercentage of maximum speed if the noise level is of particular concernto the user. Other additional or alternative preferred operatingcharacteristics of the ULT may, however, be used to define operatingparameters of the system 20.

As explained in further detail below, a plurality of sensors S₁ throughS₁₈ are each operatively coupled to the controller 130 to sensedifferent properties of the one or both of the refrigerants 34, 36 alongthe first and/or second stages 24, 26, the temperature of the ambientair surrounding the system 20, or that of the interior 16 c of cabinet16, and/or the condition of the door 16 b (i.e., open or closed) (FIG.1). These sensors are configured to generate respective signals to thecontroller 130 that are indicative of the sensed property or condition,such that the controller 130 may, in turn, generate respective commandsimpacting operation of the system 20.

When the system 20 is first started or requires restarting due, forexample, to revised cooling requirements, staging of the first andsecond stages is effected. An exemplary staging procedure or protocol isillustrated with continued reference to FIG. 2 and with furtherreference to the flow chart of FIG. 3. Block 150 represents the start ofthe staging procedure, specifically through activation (i.e., turningon) of the first compressor 50 and ends with activation (i.e., turningon) of the second compressor 70 (block 160). At block 152, thecontroller 130 receives a signal from a sensor S₁ that is configured tosense the temperature of the first refrigerant 34 at the outlet 44 b ofheat exchanger 44. At block 154, the controller 130 compares the sensedtemperature of the first refrigerant 34 to a predetermined thresholdtemperature T_(th). If the sensed temperature is less than or equal tothe threshold temperature T_(th), (block 156), the controller 130activates the second stage 26 by activating the second compressor 70(block 160). In certain forms of the invention, the controller 130 couldcause the second compressor 70 to initially operate at a lower speed andthen increase to a higher maximum speed, depending upon operatorsettings for noise control and the like.

In addition to the staging protocol illustrated in FIG. 3, the stagingprotocol may additionally include other features. For example, thestaging protocol may include, at block 152, the controller 130 receivinga signal from a sensor S₂ that is configured to sense the temperature ofambient air proximate the condenser 54 and to send a signal indicativeof the sensed temperature to the controller 130. At block 166, thecontroller 130 adjusts (i.e., increases or decreases) the thresholdtemperature T_(th) according to a predetermined algorithm (block 167)taking the sensed ambient air temperature as an input. For example, atunusually high ambient temperatures, the start up of the secondcompressor 70 could be intentionally delayed or the speed of the secondcompressor 70 upon start-up could be reduced (e.g., to about 40% ratherthan about 50% of full capacity). Additionally, or alternatively, atblock 152, the controller 130 may receive a signal from a sensor S₃ thatis configured to sense a temperature of the interior 16 c of cabinet 16and to send a signal to the controller 130 that is indicative of thesensed temperature. At block 174, the controller 130 prevents activationof the second compressor 70 if the sensed temperature is higher than apredetermined value (block 175), such that the heat exchanger 44 isprovided adequate time to cool down to a predetermined level.

Effectively, this delay (block 174) in activation of the secondcompressor 70 prevents overwhelming of the heat exchanger 44, which maybe desirable to increase the life expectancy of system 20.Alternatively, or additionally, the second compressor 70 could bestarted-up at a lower speed (e.g., about 30% −40% of capacity ratherthan 50% of capacity) in response to a higher cabinet interiortemperature.

With reference to FIG. 4, an exemplary steady-state operation of thesystem 20 is schematically illustrated. In the exemplary embodiment ofthe figure, steady-state operation mode of system 20 includessimultaneously operating both of the compressors 50, 70 most or all ofthe time. To this end, the system 20 operates under one or morealgorithms that maintain a balance between the first and second stages24, 26 such that, for example, the second stage load (from heattransferred from cabinet interior 16 c) never exceeds the maximumcapacity of the first stage 24 to remove load (i.e., heat). Thefollowing description is especially applicable when both compressors 50,70 are of the variable speed type, but can be adapted to embodiments inwhich only one compressor (e.g., the second compressor 70) is ofvariable speed and having the other compressor (e.g., the firstcompressor 50) turned on and off as required. In the event both of thecompressors 50, 70 are variable speed compressors, it is likely thatboth compressors 50, 70 will be on, with operation of one or bothcompressors being controlled to obtain a desired operatingcharacteristic.

At block 180, the controller 130 receives a signal from sensor S₁sensing the temperature of first refrigerant 34 at outlet 44 b of theheat exchanger 44. At block 182, the controller 130 varies, in responseto the signal from sensor S₁ and in accordance with a predeterminedsteady-state algorithm (block 181), the speed (e.g., the rotationalspeed in RPM) of one or both of the first or second compressors 50, 70,to thereby control, for example, the load that is transferred to thesecond stage 26. In this regard, a sensor S₄ may be configured tomonitor the speed of the second compressor 70 and to generate acorresponding signal to the controller 130 to enable controlling of thespeed of the second compressor 70.

At block 184, controller 130 determines whether a high-load condition ispresent in the system 20, for example, if the temperature of theinterior 16 c of cabinet 16 has had a step change (e.g., a sudden,relatively large increase). If such condition is detected, at block 186,the controller 130 may override the algorithm illustrated by blocks 181and 182, and replace operation of system 20 with a high-load algorithm,described in further detail below.

With continued reference to FIG. 4, the controller 130 may, in additionor as an alternative to receiving signals from sensor S₁, receive asignal (block 180) from a sensor S₅ configured to sense the dischargepressure of the second refrigerant 36 and to send a signal indicative ofthe sensed pressure to the controller 130. The sensed discharge pressureof the second refrigerant 36 may be indicative of an imbalance conditionin the system 20 caused, for example, by a high-load condition. If apredetermined pressure is sensed by sensor S₅, the controller 130 may,as explained above, (block 186), override the algorithm illustrated byblocks 181 and 182 and replace operation of system 20 with the high-loadalgorithm (block 186).

In addition, or as an alternative to the sensing provided by sensors S₁and/or S₅, one or more sensors S₆, S₇, S₈ are operatively coupled to thecontroller 130 and are respectively configured to sense a dischargepressure, discharge temperature, and/or suction temperature of the firstrefrigerant 34. Each of these sensors S₆, S₇, S₈ is configured togenerate a signal indicative of the sensed property or condition of thefirst refrigerant 34 to the controller 130 (block 180). The sensedproperty or condition of the first refrigerant 34 may be indicative ofan imbalance condition in the system caused, for example, by a high-loadcondition. If a predetermined property or characteristic is sensed byone or more of the sensors S₆, S₇, S₈, the controller 130 may, asexplained above, override the algorithm illustrated by blocks 181 and182 and replace operation of system 20 with the high-load algorithm(block 186).

As explained above, under certain conditions, the controller 130 mayoverride the algorithm (block 181) used during steady-state operation ofsystem 20 and substitute for it a high-load algorithm. In this regard,and with reference to FIGS. 4 and 5, the controller 130 may receive, atblock 180, one or more signals from various sensors of system 20, withthese signals being indicative of a high-load condition. Morespecifically, for example, a high-load condition may be present if arelatively warm item is placed in the interior 16 c of cabinet 16. Tothis end, the controller 130 may receive a signal from the sensor S₃indicative of a rise in temperature of the interior 16 c of cabinet 16.In a specific embodiment, the controller 130 may calculate a slopecorresponding to the rise in temperature of the interior 16 c over time,based on the signal from sensor S₃, and compare the same (block 194) toa predetermined threshold slope. In response to receiving this signal,and more specifically in response to the comparison, the controller may,at block 186, substitute the high-load algorithm for the steady-statealgorithm controlling operation of system 20. Under the high-loadalgorithm, in one specific embodiment, the controller 130 may increase(block 202) the speed of one or both of the compressors 50, 70.

In another example, the controller 130 may receive (block 180) a signalfrom a sensor S₉ in the form of a switch, for example, configured tosense the condition of the door 16 b of cabinet 16. In response to asignal from sensor S₉ indicating, for example, that the door 16 b isopen or closed, the controller 130 may at block 186, substitute thehigh-load algorithm for the steady-state algorithm (block 181)controlling operation of system 20. Under the high-load algorithm, asexplained above, the controller 130 may for example increase (block 202)the speed of one or both of the compressors 50, 70.

In an exemplary, yet non-limiting variation to the above-discussedprocessing of the signal received by the controller 130 from sensor S₉,the controller 130 may calculate the time the door 16 b remains in apredetermined condition (e.g., open) and compare this calculated time toa threshold value (block 194), in response to which the controllerfollows the protocol described above illustrated by blocks 186 and 202.It is contemplated that sensor S₉ may be configured instead to sense thecondition of door 16 b over a predetermined period of time, and togenerate a signal to the controller 130 that is indicative of thiscondition over the predetermined period of time, in which case thesystem 20 obviates the comparison to a threshold value otherwise carriedout by the controller 130 at block 194. For example, and withoutlimitation, an exemplary sensor S₉ capable of sensing the condition ofthe door 16 b over time may take the form of a switch and timercombination.

In yet another example, the controller 130 may receive (block 180) asignal from the sensor S₂ that is configured to sense the temperature ofambient air proximate the condenser 54 and to send a signal indicativeof the sensed temperature to the controller 130. If the received signalis indicative of a temperature that exceeds a predetermined threshold(block 194), the controller follows the protocol described aboveillustrated by blocks 186 and 202.

In addition, or as an alternative to the above, the high-load algorithmmay be triggered by the controller 130 receiving (block 180) a signalfrom the sensor S₆ indicative of a sensed discharge pressure of thefirst refrigerant 34 and/or from the sensor S₅ indicative of a senseddischarge pressure of the second refrigerant 36. In this regard, thesensed discharge pressure of the first or second refrigerants 34, 36 maybe indicative of a high-load condition and compared by the controller130 (block 194) to respective threshold pressures beyond whichcontroller 130 would follow the protocol illustrated by blocks 186 and202.

With particular reference to FIG. 5, the high-load algorithm mayinclude, in specific embodiments, increasing the speed of the fan 62directing air across the condenser 54 (block 210). This increase inspeed is facilitated by the use of a variable speed fan 62. Thisincrease in speed of fan 62 temporarily increases the rate of heattransfer from the first refrigerant 36 to the surrounding ambient 40,which results in a quicker recovery of system 20 back towards thesteady-state mode of operation. Usually, but not always, an increase inthe speed of fan 62 occurs concurrently with an increase in the speed offirst compressor 50; however, under conditions of high ambienttemperature, the speed of the fan 62 may be increased proportionatelymore than the speed of the first compressor 50. As indicated above, thespeeds of the first compressor 50 and fan 62 may be limited based onnoise control or other factors, except when extraordinary conditions aresensed.

Referring again to FIG. 2, it is contemplated that one or more othersensors S₁₀-S₁₈ may provide inputs to the controller 130 which would, inresponse to a signal received from one of the sensors S₁₀-S₁₈, vary thespeed of the fan 62, vary the speed of one or both of the compressors50, 70, or follow any of the other protocols described above. Forexample, and without limitations, a sensor S₁₀ and a sensor S₁ mayrespectively be configured to sense the sump temperature and suctiontemperature of the first refrigerant 34, while a sensor S₁₂ and a sensorS₁₃ may respectively be configured to sense the sump temperature andsuction temperature of the second refrigerant 36. Additionally oralternatively, a sensor S₁₄ may be configured to sense the dischargetemperature of the second refrigerant 36, a sensor S₁₅ may be configuredto sense the temperature of the first refrigerant 34 at an inlet 44 c tothe heat exchanger 44, a pair of sensors S₁₆, S₁₇ may be configured tosense the temperature of the second refrigerant 36 respectively at aninlet 78 a and outlet 78 b of evaporator 78, and/or a sensor S₁₈ may beconfigured to sense the speed (e.g. rotational speed, in RPM) of thefirst compressor 50. Those of ordinary skill in the art will readilyappreciate that the locations and configurations of these additionalsensors are merely exemplary rather than limiting, and it iscontemplated that other sensors may be present in system 20 in additionor as an alternative to those described above. In this regard,additional sensors may be configured to detect conditions or propertiesof system 20 or its surroundings that are not expressly describedherein, and still fall within the scope of the present disclosure.

While the present invention has been illustrated by a description ofvarious embodiments and while these embodiments have been described inconsiderable detail, it is not the intention of the applicants torestrict or in any way limit the scope of the appended claims to suchdetail. Additional advantages and modifications will readily appear tothose skilled in the art. The invention in its broader aspects istherefore not limited to the specific details, representative apparatusand method, and illustrative example shown and described. Accordingly,departures may be made from such details without departing from thespirit or scope of applicants' general inventive concept.

What is claimed is:
 1. A two-stage cascade refrigeration systemcomprising: a first refrigeration stage defining a first fluid circuitfor circulating a first refrigerant, the first refrigeration stagehaving a first compressor, a condenser, and a first expansion device influid communication with the first fluid circuit; a second refrigerationstage defining a second fluid circuit for circulating a secondrefrigerant, the second refrigeration stage having a second compressor,a second expansion device, and an evaporator in fluid communication withthe second fluid circuit; a heat exchanger in fluid communication withthe first and second fluid circuits to exchange heat between the firstand second refrigerants; wherein at least the second compressor is avariable speed compressor; at least one controller operatively coupledto the first and second compressors for independently controllingoperation thereof; and a sensor operatively coupled to the at least onecontroller, the sensor being configured to sense a temperature of thesecond refrigerant at an inlet of the evaporator and to generate asignal indicative of the sensed temperature to the at least onecontroller, the at least one controller being operable to vary a speedof the second compressor from a first speed to a second speed inresponse to the signal.
 2. The two-stage cascade refrigeration system ofclaim 1, wherein the second speed is less than the first speed.
 3. Thetwo-stage cascade refrigeration system of claim 1, wherein the secondspeed is greater than the first speed.
 4. The two-stage cascaderefrigeration system of claim 1, further comprising a variable speed fanoperable to direct air across the condenser, wherein a speed of the fanis varied from a first speed to a second speed in response to thesignal.
 5. A two-stage cascade refrigeration system comprising: a firstrefrigeration stage defining a first fluid circuit for circulating afirst refrigerant, the first refrigeration stage having a firstcompressor, a condenser, and a first expansion device in fluidcommunication with the first fluid circuit; a second refrigeration stagedefining a second fluid circuit for circulating a second refrigerant,the second refrigeration stage having a second compressor, a secondexpansion device, and an evaporator in fluid communication with thesecond fluid circuit; a heat exchanger in fluid communication with thefirst and second fluid circuits to exchange heat between the first andsecond refrigerants; wherein the first compressor and the secondcompressor are variable speed compressors; at least one controlleroperatively coupled to the first and second compressors forindependently controlling operation thereof, the first and secondcompressors being configured to operate in a steady state mode ofoperation and a high-load mode of operation; and a sensor operativelycoupled to the at least one controller, the sensor being configured tosense a temperature of the second refrigerant at an inlet to theevaporator and to generate a signal indicative of the sensed temperatureto the at least one controller, the at least one controller beingoperable to switch operation of the first and second compressors fromthe steady state mode of operation to the high-load mode of operation inresponse to the signal.
 6. The two-stage cascade refrigeration system ofclaim 5, wherein, in the steady state mode of operation, one or both ofthe first and second compressors operates at a maximum capacity.
 7. Thetwo-stage cascade refrigeration system of claim 5, wherein, in thesteady state mode of operation, one or both of the first and secondcompressors operates at less than a maximum capacity.
 8. The two-stagecascade refrigeration system of claim 5, wherein, in the high-load modeof operation, a speed of one or both of the first and second compressorsis increased from a first speed to a second speed.
 9. The two-stagecascade refrigeration system of claim 5, further comprising a variablespeed fan operable to direct air across the condenser, wherein, in thehigh-load mode of operation, a speed of the fan is increased from afirst speed to a second speed.